I had jury duty today... or maybe not. Events got me released early, and I was able to talk to some of the remaining MTA Light Rail crew at Cromwell (namely because dad's retired MTA Light Rail). Because of how light the staff is there now, MTA has put Alstom's on-site techs there to debug the rebuilt Light Rail vehicles. From what dad and I gathered:

New LR vehicles are not compatible with the pre-rebuild vehicles. You can't even couple them together.

Inside: Resin-based floor, instead of plas-steel.

Inside: Operator panels have changed considerably.

Inside: Electronics and computer systems on board are all new and incompatible.

Outside: All destination signs are LEDs and have a different font.

Outside: Any headlights or illumination are all LEDs now.

Outside: The doors no longer use air pressure, and are now electric driven.

Its not uncommon for new fleets not to be compatible with old fleets of electric MU cars. Such is the case with the new CTA 5000 cars, South Shore 300 cars, Metro North M8 and M7 cars, and SEPTA SL5 cars. None can link with predecessors for anything more than an emergency tow with no power.